This invention is directed generally to motor control circuits and more particularly to a motor control circuit for control of a motor which is used to operate a switch, such as a switch for switching of a power circuit or a power switching device such as a circuit breaker.
Generally speaking, a power switching device such as a circuit breaker may include one or more pairs of contacts and a mechanism for making or breaking these contacts. In the case of relatively high current power circuits, it may be impractical to manually throw a switch which makes or breaks the contacts of the power switching device or circuit breaker. Often, in such circuits a remotely controlled device such as a motor is used. In the case of relatively heavy circuit breakers for high current applications, the element used to close the contacts is provided with a substantial amount of closing force, for example by spring loading, to reliably close the contacts and avoid arcing, as well as to minimize contact resistance in the relatively high current circuit. Thus, a correspondingly large amount of force is required to break the contacts, that is, pull the contact closing device away from the contacts when it is desired to open the circuit, or "trip" the circuit breaker. Commonly used devices from lower current circuits such as solenoids generally do not provide the forces required to make and break such contacts in relatively high current circuits. While a relatively large handle or lever might be provided to manually make or break the contacts, because of arcing and the like associated with the making and breaking of high current circuits, it is desirable to provide a remotely controlled device. Moreover, the manual force required to be applied to such a lever may also be considerable.
Generally speaking, a motor is used in such situations. Usually, the motor is a relatively high torque motor and is provided with gearing, such that the motor revolutions are geared down to provide both the required force and the required rotary motion of a lever for making and breaking the contacts. Since only a limited amount of motion is necessary to completely make and break the contacts, usually the motor is only turned for a relatively short period of time, to achieve the revolutions and resultant motion through the associated gearing to completely make and break the contacts. In relatively high current circuits of this type, the motion provided for breaking of the contacts usually is such that the member or device for closing the contacts is moved a sufficient distance from the contacts to prevent any arcing therebetween when the contacts have been completely opened.
While various circuits have been used for motor control in the above-described situation, there is room for further improvement. Accordingly, the present invention presents a novel and improved motor control circuit for controlling a motor used to open or close one or more pairs of contacts in a power switching device such as a circuit breaker.
Moreover, in one application, it is desired to switch between line power and generator power, wherein both the line and generator comprises relatively high current power sources. However, in this application it is also desirable to delay the switching from the line to the generator until the generator has reached its operating voltage or a predetermined percentage of its operating voltage.